Hi
@CrzyChickiJules I'm sorry you're having trouble.
Do you provide a source of poultry grit for your birds? Since they are confined to coop only, it would be best to offer grit (crushed granite) free choice. This will help to process the foods that you are feeding.
For 25 birds, do you have multiple food/water stations?
I'm not berating you at all, but with the amount of birds you have in just the coop space, they are probably overheating and possibly not getting enough water/food. The space you give is around 3.5 sq.ft. per bird - this is usually o.k. if birds have access to adequate outdoor space as well. I do understand fencing and making a secure run turns into $$$. If there is any way you can get your birds outside, they will benefit from sunshine, dust bathing and be healthier/happier.
For your sick girl, it does sound like she may have a crop issue. Treatment options for crop issues are in the link below.
It's also possible that the too close for comfort quarters is stressful on the birds. Diseases like Marek's can present with birds going off feed, lethargy, immobility and death. Stress can make symptoms worse.
If you happen to lose her, then consider having a necropsy performed to make sure that you aren't dealing with an illness. Crop issues can be corrected and/or prevented, but disease will need to be dealt with in another manner.
Keep us posted.
http://www.backyardchickens.com/a/impacted-slow-and-sour-crops-prevention-and-treatments
http://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq